Studentische Arbeiten

Titel: Measurement & Compensation of Time-Variant RF-Components - Joint Project with Nokia
Typ:
  • Studienarbeit
Betreuer:
Status: offen

Hintergrund

Massive MIMO is a multi-user MIMO technology where each base station is equipped with an array of active antenna elements and utilizes these to communicate with multiple single-antenna terminals—over the same time and frequency band. By coherent processing of the signals over the array, transmit precoding can be used in the downlink to focus each signal at its desired terminal and receive combining can be used in the uplink to discriminate between signals sent from different terminals.

Problemstellung

The canonical Massive MIMO system operates in time-division duplex (TDD) mode, where the uplink and downlink transmissions take place in the same frequency resource but are separated in time. The physical propagation channels are reciprocal—meaning that the channel responses are the same in both directions—which can be utilized in TDD operation. In particular, Massive MIMO systems exploit the reciprocity to estimate the channel responses on the uplink and then use the acquired channel state information (CSI) for both uplink receive combining and downlink transmit precoding of payload data. However, the non-symmetric characteristics of the radio-frequency (RF) circuit between the transmitter and receiver generally distort the reciprocity. In order to make use of the reciprocity assumption and rely on the uplink CSI to compute precoding coefficients, the non-reciprocal transceiver responses need to be calibrated. Such a procedure is often termed reciprocity calibration. Depending on the system setup and requirements, the approach adopted can take many forms, e.g. (external) “over-the-air” measurements or (internal) daisy-chain measurements based on a dedicated hardware circuitry.

A typical assumption for calibration methods is the (quasi-) time-invariance of the non-symmetric RF characteristics, which allows for much smaller calibration rates in comparison to the CSI update rate. In TDD systems, however, RF components such as the power amplifier (PA) switch at a time scale of few milliseconds, which causes temperature variations of up to 50 Kelvin inside the PA. This yields a time-variant PA transfer-function whose gain and phase can change up to 1-2dB and 15 degrees within few milliseconds, respectively. For linear/nonlinear precoding methods that rely on accurate CSI, such time varying transfer functions can cause seriously performance losses.

Aufgabe

The goal of the student work is the characterization/compensation of time-variant components in an existing RF transceiver chain, and to investigate their impact on the system performance:

- The main focus is on the PA whose gain variation and phase drift needs to be measured/quantified.

- The results shall be used to model the time-variant RF components in an existing link-level simulator, and to quantify their impact onto the system performance. Moreover, the enhanced link-level simulator shall be used to derive tolerable limits for the RF components’ time variance.

- Based on the tolerable time-variance limits, low-complexity digital pre-distortion routines shall be designed that enable the usage of RF components with relaxed specifications.